Double metal cyanide complex catalysts are known to be extremely useful and active catalysts for the ring-opening polymerization of heterocyclic monomers such as epoxides. In particular, catalysts of this type have been employed to prepare polyether polyols derived from propylene oxide as described, for example, in U.S. Pat. Nos. 3,404,109, 3,829,505, 3,900,518, 3,941,849, 4,355,188, 5,032,671, and 4,472,560. Double metal cyanide complex catalysts are generally recognized as superior to the caustic catalysts traditionally used to produce polyether polyols for utilization in polyurethane foams, coatings, adhesives, sealants, and elastomers due to the reduced levels of unsaturation and higher functionality of the polyols obtained using such catalysts, as described in U.S. Pat. Nos. 4,239,879, 4,242,490, and 4,985,491.
The double metal cyanide complex catalysts are generally prepared by reacting a metal salt such as zinc chloride with an alkali metal hexacyanometallate such as potassium hexacyanocobaltate in aqueous solution.
Workers in the field have generally believed that to obtain a double metal cyanide complex catalyst having satisfactory performance in epoxide polymerization it is necessary to use an amount of the metal salt which is greater than that required to completely react the alkali metal hexacyanometallate. For example, U.S. Pat. No. 5,158,922 (Hinney et al.), states the "t!o obtain a double metal cyanide complex catalyst having reproducibly high polymerization activity, it is critical that an excess of the water-soluble metal salt be employed relative to the amount of metal cyanide salt." According to this reference, a portion of the excess metal salt is retained in the catalyst upon isolation and appears to function as a promoter or co-catalyst. Later, as described in U.S. Pat. No. 5,627,122, it was found that catalysts containing a relatively small excess of metal salt, particularly those prepared using an alcohol such as tert-butyl alcohol as an organic complexing agent, offered certain advantages over catalysts containing a larger excess of metal salt. However, the metal salt still must be present in excess during the catalyst synthesis since double metal cyanide complex substances that contain no metal salt "are inactive as epoxide polymerization catalysts."
While there has been considerable interest in further improving the performance of double metal cyanide complex catalysts by changing the identity of the organic complexing agent (see, for example, U.S. Pat. No. 5,470,813) or by modifying the catalyst with a polyether (see, for example, U.S. Pat. No. 5,482,908) or zinc sulfate and/or acid (see, for example U.S. Pat. No. 4,472,560), the preparation of such catalysts using cyanide-free compounds of Group IIA elements such as calcium chloride has not heretofore been described.